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10.21: Mammal Reproduction

Difficulty Level: At Grade Created by: CK-12
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Do mammals lay eggs?

Could a rabbit really lay eggs? Rabbits are mammals, and most mammals do not lay eggs. Mammals usually give birth to live offspring. But there are some mammals that do lay eggs.

Mammal Reproduction

You probably realize that cats, dogs, people, and other mammals don't typically lay eggs. There are exceptions, however. Egg-laying is possible among the monotremes, mammals with birdlike and reptilian characteristics. Recall that mammals can be classified into three general groups, based on their reproductive strategy: the monotremes, the marsupials and the placental mammals.

The egg-laying monotremes, such as echidnas (Figure below) and platypuses (Figure below), use one opening, the cloaca, to urinate, release waste, and reproduce, just like birds. They lay leathery eggs, similar to those of lizards, turtles, and crocodilians. Monotremes feed their young by “sweating” milk from patches on their bellies, as they lack the nipples present on other mammals.

A Echidna and Platypus are both mammals, but lay eggs

The echidna (right) is a member of the monotremes, the most primitive order of mammals. Another monotreme, the platypus (left), like other mammals in this order, lays eggs and has a single opening for the urinary, genital, and digestive organs.

All other mammals give birth to live young and belong to one of two different categories, the marsupials and the placental mammals. A marsupial is an animal in which the embryo, which is often called a joey, is born at an immature stage. Development must be completed outside the mother's body. Most female marsupials have an abdominal pouch or skin fold where there are mammary glands. The pouch is a place for completing the development of the baby. Although blind, without fur, and with only partially formed hind legs, the tiny newborns have well developed forelimbs with claws that enable them to climb their way into their mother's pouch where they drink their mother's milk and continue their development. Marsupials include kangaroos, koalas, and opossums. Other marsupials are the wallaby and the Tasmanian Devil. Most marsupials live in Australia and nearby areas. (Figure below).

The majority of mammals are placental mammals. These are mammals in which the developing baby is fed through the mother's placenta. Female placental mammals develop a placenta after fertilization. A placenta is a spongy structure that passes oxygen, nutrients, and other useful substances from the mother to the fetus. It also passes carbon dioxide and other wastes from the fetus to the mother. The placenta allows the fetus to grow for a long time within the mother.

A kangaroo is a marsupial animal

A marsupial mammal, this eastern gray kangaroo has a joey (young kangaroo) in its abdominal pouch.

Some mammals are alone until a female can become pregnant. Others form social groups with big differences between sexes, such as size differences, a trait called sexual dimorphism. Dominant males are those that are the largest or best-armed. These males usually have an advantage in mating. They may also keep other males from mating with females within a group. This is seen in elephant seals (Figure below), and also with elk, lions and non-human primates, including the orangutans and gorillas. Male elk grow antlers, while female elk do not have antlers. Adult male lions are not only larger than females, they have a mane of long hair on the side of the face and top of the head.

Seals have a mating system with many females and one male

A mating system with a group of many females and one male, as seen in the seal species. Male elephant seals can grow to 14 feet long, whereas females can grow to 11 feet long.


  • Monotremes can lay eggs, but most mammals give birth to live young.
  • Mammals can be marsupial, where the embryo is born at an immature stage and develops in the pouch.
  • Mammals can be placental, where substances are passed from the mother to the fetus so that it can stay longer in the womb.

Explore More

Use the resources below to answer the questions that follow.

Explore More I

  1. What are four examples of marsupials?
  2. Where are most marsupials found?
  3. What is the primary difference between marsupial and placental mammals? Explain your response.

Explore More II

  1. How does parental care in marsupial mammals differ from parental care in placental mammals?
  2. Why do marsupials lick themselves during birth?
  3. How do scientists think marsupial babies find the pouch?

Explore More III

  1. What are two of the purposes of the placenta?
  2. What does the placenta allow to pass to the embryo?
  3. What does the placenta keep from the embryo?
  4. What is the function of the umbilical cord?


  1. How are monotremes more like reptiles and birds than other mammals?
  2. What's the difference between marsupial and placental animals?
  3. What are three marsupials?
  4. Define and describe the role of the placenta.
  5. What is sexual dimorphism?

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cloaca Single exit and entrance for sperm, eggs, and waste.
marsupial Mammal in which the young are born in an immature state and continue development in the pouch.
monotreme Mammal with some birdlike and reptilian characteristics, such as laying eggs.
placenta Spongy structure that passes oxygen, nutrients, and other useful substances from the mother to the fetus.
placental Mammal with an organ that feeds the fetus and removes waste products.
sexual dimorphism Distinct difference in appearance between the males and females.

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Difficulty Level:
At Grade
7 , 8
Date Created:
Nov 29, 2012
Last Modified:
Aug 30, 2016
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